Bump celery from 4.3.0 to 4.4.0

Bumps [celery](https://github.com/celery/celery) from 4.3.0 to 4.4.0.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog.rst)
- [Commits](https://github.com/celery/celery/compare/v4.3.0...4.4.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -4,7 +4,7 @@ sentry-sdk==0.14.1
 
 # Production dependencies
 boto3==1.9.245
-celery==4.3.0
+celery==4.4.0
 dj-database-url==0.5.0
 django-anymail==6.0.1
 django-basic-auth-ip-whitelist==0.3.1